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Thursday, February 25, 2010

The coast of present-day Massachusetts was inhabited by several Native American tribes in the early 17th century. Fishermen, traders, and explorers from the Old World encountered the Indians during their occasional travel through the area. However by the time the Mayflower landed in Plymouth in 1620 to establish a colony, a mysterious epidemic had ravaged coastal New England, killing up to 90% of the indigenous population during the years 1616 through 1619. Experts have yet to agree on the cause of the epidemic. Smallpox, plague, and yellow fever, all highly lethal diseases, have been blamed.

An article in the new issue of Emerging Infectious Diseases offers leptospirosis, caused by Leptospira spirochetes, as another possible agent of the 1616-1619 epidemic. This is based not on any new information but on an examination of the lifestyle of the Native Americans of early 17th century New England.

Rats infected with Leptospira may have stowed away in the ships that sailed from Europe to the New World. Because Leptospira lives in the kidney tubules of chronic carriers, infected rats released into the New World would have contaminated their surroundings every time they urinated. Since Leptospira can survive in moist soil and fresh water, indigenous rodents and other animals could have become chronically infected with Leptospira, further spreading the spirochete throughout the region. The Indian lifestyle provided plenty of opportunities for exposure to Leptospira through skin abrasions and swallowing of contaminated water or food. Their high-risk activities included the following:

walking around barefooted

storing food accessible to rodents

swimming and bathing in streams and ponds

working on moist soil to raise and harvest crops

Leptospira has little effect on the health of carrier animals yet can cause humans to fall ill. Many escape with what may be confused with a mild case of the flu, but some end up suffering with life-threatening symptoms. Eyewitnesses of the 1616-1619 epidemic reported that victims were afflicted with skin lesions, severe headaches, yellowing of the skin (likely jaundice), and bloody nose (possibly from lung hemorrhage), which are all symptoms of the severe form of leptospirosis. Even today leptospirosis can be deadly with reported fatality rates of greater than 50% among those with severe lung hemorrhaging.

While the authors should be commended for even considering a disease of a spirochete that is often ignored (at least by those in the developed world), I don't think Leptospira is what killed off the Indians. One strong argument against leptospirosis being the cause of the 1616-1619 epidemic is that Leptospira is not hardy enough to survive the cold winters that Mother Nature inflicts upon New England. Since the fatalities continued through the winter, leptospirosis is unlikely to be the culprit.

Whatever the cause, the epidemic may have been a pivotal event that facilitated English colonization of coastal Massachusetts since the surviving Indians lacked the capacity to resist the newcomers.

Friday, February 12, 2010

The Lyme disease agent Borrelia burgdorferi possesses flagella, which are the thin motility structures owned by many members of the bacteria world. Flagella propel bacteria towards their destination by spinning (read this post to see how flagella function in Borrelia). It has been assumed B. burgdorferi spin their flagella whenever they need to move from one location to another. A recent paper in The Journal of Clinical Investigation has demonstrated otherwise, at least for B. burgdorferi in the midgut of a feeding Ixodes (blacklegged) tick.

Borrelia burgdorferi spends much of its life cycle lying dormant in the midgut of Ixodes ticks. The spirochetes lightly pepper the inner surface of the midgut cell lining, with a few spirochetes also hiding between cells. None live at the base of the cells at the basement membrane surrounding the midgut. The spirochetes wake up and multiply only when the tick attaches to an animal or human and imbibes blood. A few days into the blood meal, some spirochetes eventually breech the basement membrane and enter the hemocoel, the fluid-filled space between the tick organs where they must avoid the phagocytes patrolling the area. From there the spirochetes invade the salivary glands, which can then release B. burgdorferi-tainted saliva into the skin of the victim. After completing its satisfying meal of blood, the tick detaches from the skin of the victim, who may end up suffering from Lyme disease.

Dunham-Ems and colleagues wanted to follow the spirochetes in the midgut as ticks took their meal of blood. They engineered a strain of B. burgdorferi expressing green fluorescent protein so that they could watch the spirochetes in the gut by fluorescence microscopy. They allowed ticks with the green B. burgdorferi strain in their midguts to feed on laboratory mice. 24, 48, and 72 hours after the ticks were placed on the mice, the investigators removed the midguts and examined the organ by fluorescence microscopy to see what the spirochetes were doing. Surprisingly, they never saw motile spirochetes in the midgut even though the spirochetes eventually found their way at 72 hours into the hemocoel, where they were highly motile.

If the spirochetes in the midgut remained nonmotile during tick feeding, how did they reach the basement membrane? The few spirochetes that initially populated the midgut multiplied exponentially and formed growing networks of spirochetes on the cell surfaces as the tick drank blood from the mice. By 72 hours the networks eventually coalesced, encasing many gut cells in spirochetes (see the figures below). Spirochetes at the base of the encased cells were poised to penetrate the basement membrane and invade the hemocoel. All of this happened without B. burgdorferi ever spinning its flagella. Only when they broke through into the hemocoel did the flagella start spinning.

Figure 4F-H from Dunham-Ems 2009. Confocal fluorescence microscopy of a midgut from a nymph that fed on a mouse for 72 hours. Panel F shows a network of spirochetes (green) attached to the inner surface of the midgut. An optical section taken 24-26 µm into the lining of the midgut (panel G) reveals aggregates of spirochetes surrounding the cells. Panel H shows that some spirochetes have made it to the basement membrane, which is found 50 µm below the surface. The midgut cell membrane is stained in red. Scale bars = 25 µm. Some of the gut cells are extremely large because they are differentiating as part of the digestion process.

Figure 5 A and B from Dunham-Ems 2009. Silver stain of sections from ticks that fed for 48 hours (panel A) and 72 hours (panel B). The edges of the epithelial cells are easier to see than in the previous figure. Arrows point to aggregates of spirochetes (hairy bodies). At 72 hours at least one cell is encased in spirochetes. Scale bars = 25 µm. Some of the cells are extremely large because they are differentiating as part of the normal digestion process of the tick (dc, differentiated cells; uc, undifferentiated cells).

The investigators also found that something in the tick midgut inhibited the motility of B. burgdorferi. They placed a bit of minced midgut from a tick that had been feeding on a mouse for 72 hours at the edge of a gelatin matrix containing motile fluorescent B. burgdorferi. (Because of their helical shape, spirochetes love to move about in viscous substances such as gelatin.) Most of the spirochetes near the tissue ceased moving and remained motionless throughout the 15 minute viewing period. In contrast, the spirochetes continued moving when mouse blood was placed at the edge of the gelatin matrix.

Why does B. burgdorferi employ a nonmotile mode of penetration of the cell lining of the tick midgut? Is there some advantage for the spirochete to avoid using their flagella? As blood is known to be a powerful chemoattractant for B. burgdorferi, the authors offered the following explanation:

These results, although counterintuitive at first blush, make sense; if blood in the midgut acted as a chemoattractant, spirochetes would never disseminate during feeding.

Hence the "inhibitor" of motility released by the tick gut serves as a signal to the spirochete to not spin their flagella.

To me, this explanation isn't satisfying. It would seem simple for B. burgdorferi to have evolved a regulatory scheme that would allow the spirochete to temporarily uncouple blood chemotaxis from flagellar motility so that they could bore through the gut lining in minutes rather than days. There must be a reason why B. burgdorferi chooses to take its time to penetrate the gut lining.

Perhaps B. burgdorferi delays its journey to the salivary glands to allow the feeding tick to properly prepare the skin, which is an inhospitible environment for both tick and spirochete. As the tick feeds, it releases a brew of anti-immune factors into the skin to protect itself from attack by the immune system. Early arrival of B. burgdorferi to the salivary gland would release the spirochetes into the skin before the anti-immune factors have taken full effect, potentially allowing the host immune system to eliminate the spirochetes before they could establish an infection.

Feeding Ixodes ticks harboring Borrelia burgdorferi deposit the Lyme disease spirochete in the skin of the victim. The spirochetes remain...

Common Spirochete Diseases

Lyme disease is a tick-borne disease caused by several members of the Borrelia burgdorferi complex. B. burgdorferi, B. garinii, and B. afzelii account for most cases worldwide. A rash may appear at the site of the tick bite, and the patient may experience flu-like symptoms. Left untreated, the patient may suffer from neurologic, arthritic, and cardiac complications.

The syphilis agent Treponema pallidum is most commonly acquired by sexual contact. A skin lesion called a chancre appears at the site of initial contact with the spirochete. T. pallidum later spreads to other sites in the body to cause the flu-like symptoms and rash of secondary syphilis. Once secondary syphilis resolves, the spirochete may persist for years without causing problems. Later, tertiary syphilis can result in damage to vital tissues. Neurosyphilis and cardiovascular syphilis are two common forms of tertiary syphilis.

Leptospira lives in the kidneys of rodents and other reservoir hosts and is shed via urine into the environment. Humans acquire the spirochete by contact of abraded skin or mucous membranes with infectious urine or contaminated water or soil. Leptospirosis patients may initially experience flu-like symptoms. Jaundice and impaired kidney function occur in the potentially deadly form of leptospirosis called Weil's disease.